In an optical communication system, an optical amplifying apparatus is used as an optical repeater in which a signal light is amplified directly by an optical amplifier. An output of the optical amplifying apparatus is required to be maintained at a constant level so that the output level thereof is monitored continuously by monitoring a power level of a light which is obtained directly by dividing the output of the optical amplifying apparatus. However, the output light includes a spontaneous light emitted from the optical amplifier as well as the optical signal so that the monitoring is affected by the spontaneous light. In order to remove the influence of the spontaneous light, a low frequency signal which is monitored is added to a signal light. Then, some part of the amplified light is divided to be converted to an electric signal for the monitoring purpose. This system has been described on pages 4 to 38 of the proceeding paper of the Autumn National Assembly of Electronic Information Communication Society, vol. 4, 1989, entitled, "Examination of a semiconductor laser optical amplifying repeater."
The conventional amplifying apparatus, however, has a disadvantage in that the wavelength dispersion becomes large in an optical fiber, because there occurs a so called "wavelength chirping" in which the frequency of the signal light is modulated by adding the low frequency signal to the signal light. Such a wavelength dispersion causes a transmission length to be shortened in the optical communication.